Heterologous Expression of Plantaricin 423 and Mundticin ST4SA in Saccharomyces cerevisiae

Antimicrobial peptides or bacteriocins are excellent candidates for alternative antimicrobials, but high manufacturing costs limit their applications. Recombinant gene expression offers the potential to produce these peptides more cost-effectively at a larger scale. Saccharomyces cerevisiae is a popular host for recombinant protein production, but with limited success reported on antimicrobial peptides. Individual recombinant S. cerevisiae strains were constructed to secrete two class IIa bacteriocins, plantaricin 423 (PlaX) and mundticin ST4SA (MunX). The native and codon-optimised variants of the plaA and munST4SA genes were cloned into episomal expression vectors containing either the S. cerevisiae alpha mating factor (MFα1) or the Trichoderma reesei xylanase 2 (XYNSEC) secretion signal sequences. The recombinant peptides retained their activity and stability, with the MFα1 secretion signal superior to the XYNSEC secretion signal for both bacteriocins. An eight-fold increase in activity against Listeria monocytogenes was observed for MunX after codon optimisation, but not for PlaX-producing strains. After HPLC-purification, the codon-optimised genes yielded 20.9 mg/L of MunX and 18.4 mg/L of PlaX, which displayed minimum inhibitory concentrations (MICs) of 108.52 nM and 1.18 µM, respectively, against L. monocytogenes. The yields represent a marked improvement relative to an Escherichia coli expression system previously reported for PlaX and MunX. The results demonstrated that S. cerevisiae is a promising host for recombinant bacteriocin production that requires a simple purification process, but the efficacy is sensitive to codon usage and secretion signals.

Enzymatic hydrolysis of single-use bioplastic items by improved recombinant yeast strains

Single-use bioplastic items pose new challenges for a circular plastics economy as they require different processing than petroleum-based plastics items. Microbial and enzymatic recycling approaches could address some of the pitfalls created by the influx of bioplastic waste. In this study, the recombinant expression of a cutinase-like-enzyme (CLE1) was improved in the yeast Saccharomyces cerevisiae to efficiently hydrolyse several commercial single-use bioplastic items constituting blends of poly(lactic acid), poly(1,4-butylene adipate-co-terephthalate), poly(butylene succinate) and mineral fillers. The hydrolysis process was optimised in controlled bioreactor configurations to deliver substantial monomer concentrations and, ultimately, 29 to 78% weight loss. Product inhibition studies and molecular docking provided insights into potential bottlenecks of the enzymatic hydrolysis process, while FT-IR analysis showed the preferential breakdown of specific polymers in blended commercial bioplastic items. This work constitutes a step towards implementing enzymatic hydrolysis as a circular economy approach for the valorisation of end-of-life single-use bioplastic items.

Additional glucoamylase genes increase ethanol productivity on rice and potato waste streams by a recombinant amylolytic yeast

The implementation of consolidated bioprocessing for converting starch to ethanol relies on a robust yeast that produces enough amylases for rapid starch hydrolysis. Furthermore, using low-cost substrates will assist with competitive ethanol prices and support a bioeconomy, especially in developing countries. This paper addresses both challenges with the expression of additional glucoamylase gene copies in an efficient amylolytic strain (Saccharomyces cerevisiae ER T12) derived from the industrial yeast, Ethanol Red™. Recombinant ER T12 was used as a host to increase ethanol productivity during raw starch fermentation; the ER T12.7 variant, selected from various transformants, displayed enhanced raw starch conversion and a 36% higher ethanol concentration than the parental strain after 120 h. Unripe rice, rice bran, potato waste and potato peels were evaluated as alternative starchy substrates to test ER T12.7's fermenting ability. ER T12.7 produced high ethanol yields at significantly improved ethanol productivity, key criteria for its industrial application.

Promoters and introns as key drivers for enhanced gene expression in Saccharomyces cerevisiae

The transcription of genes in the yeast Saccharomyces cerevisiae is governed by multiple layers of regulatory elements and proteins, cooperating to ensure optimum expression of the final protein product based on the cellular requirements. Promoters have always been regarded as the most important determinant of gene transcription, but introns also play a key role in the expression of intron-encoding genes. Some introns can enhance transcription when introduced either promoter-proximal or embedded in the open reading frame of genes. However, the outcome is seldom predictable, with some introns increasing or decreasing transcription depending on the promoter and reporter gene employed. This chapter provides an overview of the general structure and function of promoters and introns and how they may cooperate during transcription to allow intron-mediated enhancement of gene expression. Since S. cerevisiae is a suitable host for recombinant protein production on a commercial level, stronger and more controllable promoters are in high demand. Enhanced gene expression can be achieved via promoter engineering, which may include introns that increase the efficacy of recombinant expression cassettes. Different models for the role of introns in transcription are briefly discussed to show how these intervening sequences can actively interact with the transcription machinery. Furthermore, recent examples of improved protein production via the introduction of promoter-proximal introns are highlighted to showcase the potential value of intron-mediated enhancement of gene expression.

Engineered yeast for the efficient hydrolysis of polylactic acid

Polylactic acid (PLA) is a major contributor to the global bioplastic production capacity. However, post-consumer PLA waste is not fully degraded during non-optimal traditional organic waste treatment processes and can persist in nature for many years. Efficient enzymatic hydrolysis of PLA would contribute to cleaner, more energy-efficient, environmentally friendly waste management processes. However, high costs and a lack of effective enzyme producers curtail the large-scale application of such enzymatic systems. This study reports the recombinant expression of a fungal cutinase-like enzyme (CLE1) in the yeast Saccharomyces cerevisiae, which produced a crude supernatant that efficiently hydrolyses different types of PLA materials. The codon-optimised Y294[CLEns] strain delivered the best enzyme production and hydrolysis capabilities, releasing up to 9.44 g/L lactic acid from 10 g/L PLA films with more than 40% loss in film weight. This work highlights the potential of fungal hosts producing PLA hydrolases for future commercial applications in PLA recycling.

Supplementation of recombinant cellulases with LPMOs and CDHs improves consolidated bioprocessing of cellulose

The increased demand for energy has sparked a global search for renewable energy sources that could partly replace fossil fuel resources and help mitigate climate change. Cellulosic biomass is an ideal feedstock for renewable bioethanol production, but the process is not currently economically feasible due to the high cost of pretreatment and enzyme cocktails to release fermentable sugars. Lytic polysaccharide monooxygenases (LPMOs) and cellobiose dehydrogenases (CDHs) are auxiliary enzymes that can enhance cellulose hydrolysis. In this study, four LPMO and two CDH genes were subcloned and expressed in the Saccharomyces cerevisiae Y294 laboratory strain. SDS-PAGE analysis confirmed the extracellular production of the LPMOs and CDHs in the laboratory S. cerevisiae Y294 strain. A rudimentary cellulase cocktail (cellobiohydrolase 1 and 2, endoglucanase and β-glucosidase) was expressed in the commercial CelluX™ 4 strain and extracellular production of the individual cellulases was confirmed by SDS-PAGE analysis. In vitro cooperation of the CDHs and LPMOs with the rudimentary cellulases produced by strain CelluX™ 4[F4-1] was demonstrated on Whatman filter paper. The significant levels of soluble sugars released from this crystalline cellulose substrate indicated that these auxiliary enzymes could be important components of the CBP yeast cellulolytic system.

Constructing recombinant Saccharomyces cerevisiae strains for malic-to-fumaric acid conversion

Saccharomyces cerevisiae with its robustness and good acid tolerance, is an attractive candidate for use in various industries, including waste-based biorefineries where a high-value organic acid is produced, such as fumaric acid could be beneficial. However, this yeast is not a natural producer of dicarboxylic acids, and genetic engineering of S. cerevisiae strains is required to achieve this outcome. Disruption of the natural FUM1 gene and the recombinant expression of fumarase and malate transporter genes improved the malic acid-to-fumaric acid conversion by engineered S. cerevisiae strains. The efficacy of the strains was significantly influenced by the source of the fumarase gene (yeast versus bacterial), the presence of the XYNSEC signal secretion signal and the available oxygen in synthetic media cultivations. The ΔFUM1Ckr_fum + mae1 and ΔFUM1(ss)Ckr_fum + mae1 strains converted extracellular malic acid into 0.98 and 1.11 g/L fumaric acid under aerobic conditions.

Promoter-proximal introns impact recombinant amylase expression in Saccharomyces cerevisiae

Consolidated bioprocessing (CBP) of starch requires recombinant Saccharomyces cerevisiae strains that produce raw starch-degrading enzymes and ferment the resultant sugars to ethanol in a single step. In this study, the native S. cerevisiae COX4 and RPS25A promoter-proximal introns were evaluated for enhanced expression of amylase genes (ateA, temA or temG_Opt) under the control of an S. cerevisiae promoter (ENO1P, TEF1P, TDH3P, or HXT7P). The results showed that different promoters and promoter-intron combinations differentially affected recombinant amylase production: ENO1P-COX4i and TDH3P-RPS25Ai were the best promoters for AteA, followed closely by HXT7P. The latter was also the best promoter for TemA and TemG production, followed closely by TDH3P-RPS25Ai for both these enzymes. Introducing promoter-proximal introns increased amylase activity up to 62% in Y294[ENO-COX-AteA] and Y294[TDH3-RPS-TemA], a significant improvement relative to the intron-less promoters. Strains co-expressing both an α-amylase and glucoamylase genes yielded up to 56 g/L ethanol from 20% w/v raw starch, with a higher carbon conversion observed with strains co-expressing TDH3P-RPS25Ai-temG_Opt than HXT7P-temG_Opt. The study showed that promoter-proximal introns can enhance amylase activity in S. cerevisiae and suggest that these alternative cassettes may also be considered for expression in more efficient ethanol-producing industrial yeast strains for raw starch CBP.

Medium optimization for enhanced production of recombinant lignin peroxidase in Pichia pastoris

OBJECTIVES

Different cultivation conditions and parameters were evaluated to improve the production and secretion of a recombinant Phanerochaete chrysosporium lipH8 gene in Komagataella phaffii (Pichia pastoris).

RESULTS

The recombinant lipH8 gene with its native secretion signal was successfully cloned and expressed in Komagataella phaffii (Pichia pastoris) under the control of the alcohol oxidase 1 promoter (P_AOX1). The results revealed that co-feeding with sorbitol and methanol increased rLiP secretion by 5.9-fold compared to the control conditions. The addition of 1 mM FeSO₄ increased LiP activity a further 6.0-fold during the induction phase. Moreover, the combination of several optimal conditions and parameters yielded an extracellular rLiP activity of 20.05 U l^-1, which is more than ten-fold higher relative to standard growth conditions (BMM10 medium, pH 6 and 30 °C).

CONCLUSION

Extracellular activity of a recombinant LiP expressed in P. pastoris increased more than ten-fold when co-feeding sorbitol and methanol as carbon sources, together with urea as nitrogen source, FeSO₄ supplementation, lower pH and lower cultivation temperature.

Heterologous production of cellulose- and starch-degrading hydrolases to expand Saccharomyces cerevisiae substrate utilization: Lessons learnt

Selected strains of Saccharomyces cerevisiae are used for commercial bioethanol production from cellulose and starch, but the high cost of exogenous enzymes for substrate hydrolysis remains a challenge. This can be addressed through consolidated bioprocessing (CBP) where S. cerevisiae strains are engineered to express recombinant glycoside hydrolases during fermentation. Looking back at numerous strategies undertaken over the past four decades to improve recombinant protein production in S. cerevisiae, it is evident that various steps in the protein production "pipeline" can be manipulated depending on the protein of interest and its anticipated application. In this review, we briefly introduce some of the strategies and highlight lessons learned with regards to improved transcription, translation, post-translational modification and protein secretion of heterologous hydrolases. We examine how host strain selection and modification, as well as enzyme compatibility, are crucial determinants for overall success. Finally, we discuss how lessons from heterologous hydrolase expression can inform modern synthetic biology and genome editing tools to provide process-ready yeast strains in future. However, it is clear that the successful expression of any particular enzyme is still unpredictable and requires a trial-and-error approach.

Evaluating and engineering Saccharomyces cerevisiae promoters for increased amylase expression and bioethanol production from raw starch

Bioethanol production from starchy biomass via consolidated bioprocessing (CBP) will benefit from amylolytic Saccharomyces cerevisiae strains that produce high levels of recombinant amylases. This could be achieved by using strong promoters and modification thereof to improve gene expression under industrial conditions. This study evaluated eight endogenous S. cerevisiae promoters for the expression of a starch-hydrolysing α-amylase gene. A total of six of the native promoters were modified to contain a promoter-proximal intron directly downstream of the full-length promoter. Varying results were obtained; four native promoters outperformed the ENO1P benchmark under aerobic conditions and two promoters showed better expression under simulated CBP conditions. The addition of the RPS25A intron significantly improved the expression from most promoters, displaying increased transcript levels, protein concentrations and amylase activities. Raw starch-utilising strains were constructed through co-expression of selected α-amylase cassettes and a glucoamylase gene. The amylolytic strains displayed improved fermentation vigour on raw corn starch and broken rice, reaching 97% of the theoretical ethanol yield and converting 100% of the available carbon to products within 120 h in small-scale CBP fermentations on broken rice. This study showed that enhanced amylolytic strains for the conversion of raw starch to ethanol can be achieved through turnkey promoter selection and/or engineering.

Consolidated bioprocessing of raw starch to ethanol by Saccharomyces cerevisiae: Achievements and challenges

Recent advances in amylolytic strain engineering for starch-to-ethanol conversion have provided a platform for the development of raw starch consolidated bioprocessing (CBP) technologies. Several proof-of-concept studies identified improved enzyme combinations, alternative feedstocks and novel host strains for evaluation and application under fermentation conditions. However, further research efforts are required before this technology can be scaled up to an industrial level. In this review, different CBP approaches are defined and discussed, also highlighting the role of auxiliary enzymes for a supplemented CBP process. Various achievements in the development of amylolytic Saccharomyces cerevisiae strains for CBP of raw starch and the remaining challenges that need to be tackled/pursued to bring yeast raw starch CBP to industrial realization, are described. Looking towards the future, it provides potential solutions to develop more cost-effective processes that include cheaper substrates, integration of the 1G and 2G economies and implementing a biorefinery concept where high-value products are also derived from starchy substrates.

Synergistic codon optimization and bioreactor cultivation toward enhanced secretion of fungal lignin peroxidase in Pichia pastoris: Enzymatic valorization of technical (industrial) lignins

Lignin peroxidase (LiP) is a well-recognized enzyme for its ability to oxidize lignins, but its commercial availability is limited, which hinders the biotechnological application of LiP-based bioprocesses in lignocellulose biorefineries. This study evaluated a combination strategy to improve the expression of LiP to promote its practical use. The strategy included optimization of the lipH8 gene of Phanerochaete chrysosporium according to the codon usage of Pichia pastoris, followed by fed-batch fermentation using a 14 L bioreactor (10 L working volume). The combination strategy achieved a maximum volumetric LiPH8 activity of 4480 U L^-1, protein concentration of 417 mg L^-1 and a specific activity of 10.7 U mg^-1, which was higher than previous reports. Biochemical characterization showed that the recombinant LiPH8 (rLiPH8) was optimum at pH 3.0, 25 ℃ and 0.4 mM H₂O₂. Using the optimized conditions, rLiPH8 was used to treat isolated technical lignins namely soda-anthraquinone (SAQ) lignin and steam explosion (S-E) lignin. High-performance gel permeation chromatography (HP-GPC) analysis showed that the molecular weight (Mw) of SAQ and S-E lignins were increased by 1.43-and 1.14-fold, respectively, after the enzymatic treatment. Thermogravimetric analysis (TGA) also showed that the thermal stability of the lignins was improved, indicating that the enzyme treatment of lignins with rLiPH8 resulted in lignin re-polymerization. As the first report on rLiPH8 production using P. pastoris, this study has shed light on the possible route for the enhancement of rLiPH8 production and its potential application for upgrading technical lignins.

Consolidated bioprocessing of raw starch with Saccharomyces cerevisiae strains expressing fungal alpha-amylase and glucoamylase combinations

Cost-effective consolidated bioprocessing (CBP) of raw starch for biofuel production requires recombinant Saccharomyces cerevisiae strains expressing α-amylases and glucoamylases. Native Aureobasidium pullulans apuA, Aspergillus terreus ateA, Cryptococcus sp. S-2 cryA and Saccharomycopsis fibuligera sfiA genes encoding raw-starch α-amylases were cloned and expressed in the S. cerevisiae Y294 laboratory strain. Recombinant S. cerevisiae Y294[ApuA] and Y294[AteA] strains produced the highest extracellular α-amylase activities (2.17 U mL-1 and 2.98 U mL-1, respectively). Both the ApuA and AteA α-amylases displayed a preference for pH 4 to 5 and retained more than 75% activity after 5 days at 30°C. When ateA was co-expressed with the previously reported Aspergillus. tubingensis glucoamylase gene (glaA), the amylolytic S. cerevisiae Y294[AteA-GlaA] strain produced 45.77 g L-1 ethanol after 6 days. Ethanol production by this strain was improved with the addition of either 2.83 μL STARGEN 002 (54.54 g L-1 ethanol and 70.44% carbon conversion) or 20 μL commercial glucoamylase from Sigma-Aldrich (73.80 g L-1 ethanol and 90.19% carbon conversion). This is the first report of an engineered yeast strain that can replace up to 90% of the enzymes required for raw starch hydrolysis, and thus contributes to the realisation of a CBP yeast for starch-based biofuel production.

Improved raw starch amylase production by Saccharomyces cerevisiae using codon optimisation strategies

Amylases are used in a variety of industries that have a specific need for alternative enzymes capable of hydrolysing raw starch. Five α-amylase and five glucoamylase-encoding genes were expressed in the Saccharomyces cerevisiae Y294 laboratory strain to select for recombinant strains that best hydrolysed raw corn starch. Gene variants of four amylases were designed using codon optimisation and different secretion signals. The significant difference in activity levels among the gene variants confirms that codon optimisation of fungal genes for expression in S. cerevisiae does not guarantee improved recombinant protein production. The codon-optimised glucoamylase variant from Talaromyces emersonii (temG_Opt) yielded 3.3-fold higher extracellular activity relative to the native temG, whereas the codon-optimised T. emersonii α-amylase (temA_Opt) yielded 1.6-fold more extracellular activity than the native temA. The effect of four terminator sequences was also investigated using temG and temG_Opt as reporter genes, with the ALY2T terminator resulting in a 14% increase in glucoamylase activity relative to the gene cassettes containing the ENO1T terminator. This is the first report of engineered S. cerevisiae strains to express T. emersonii amylase variants, and these enzymes may have potential applications in the industrial conversion of raw starch under fermentation conditions.

Enrichment of maize and triticale bran with recombinant Aspergillus tubingensis ferulic acid esterase

Ferulic acid is a natural antioxidant found in various plants and serves as a precursor for various fine chemicals, including the flavouring agent vanillin. However, expensive extraction methods have limited the commercial application of ferulic acid, in particular for the enrichment of food substrates. A recombinant Aspergillus tubingensis ferulic acid esterase Type A (FAEA) was expressed in Aspergillus niger D15#26 and purified with anion-exchange chromatography (3487 U/mg, K_m  = 0.43 mM, K_cat = 0.48/min on methyl ferulate). The 36-kDa AtFAEA protein showed maximum ferulic acid esterase activity at 50 °C and pH 6, suggesting potential application in industrial processes. A crude AtFAEA preparation extracted 26.56 and 8.86 mg/g ferulic acid from maize bran and triticale bran, respectively, and also significantly increased the levels of p-coumaric and caffeic acid from triticale bran. The cost-effective production of AtFAEA could therefore allow for the enrichment of brans generally used as food and fodder, or for the production of fine chemicals (such as ferulic and p-coumaric acid) from plant substrates. The potential for larger-scale production of AtFAEA was demonstrated with the A. niger D15[AtfaeA] strain yielding a higher enzyme activity (185.14 vs. 83.48 U/ml) and volumetric productivity (3.86 vs. 1.74 U/ml/h) in fed-batch than batch fermentation.

Consolidated bioprocessing of starchy substrates into ethanol by industrial Saccharomyces cerevisiae strains secreting fungal amylases

The development of a yeast strain that converts raw starch to ethanol in one step (called Consolidated Bioprocessing, CBP) could significantly reduce the commercial costs of starch-based bioethanol. An efficient amylolytic Saccharomyces cerevisiae strain suitable for industrial bioethanol production was developed in this study. Codon-optimized variants of the Thermomyces lanuginosus glucoamylase (TLG1) and Saccharomycopsis fibuligera α-amylase (SFA1) genes were δ-integrated into two S. cerevisiae yeast with promising industrial traits, i.e., strains M2n and MEL2. The recombinant M2n[TLG1-SFA1] and MEL2[TLG1-SFA1] yeast displayed high enzyme activities on soluble and raw starch (up to 8118 and 4461 nkat/g dry cell weight, respectively) and produced about 64 g/L ethanol from 200 g/L raw corn starch in a bioreactor, corresponding to 55% of the theoretical maximum ethanol yield (g of ethanol/g of available glucose equivalent). Their starch-to-ethanol conversion efficiencies were even higher on natural sorghum and triticale substrates (62 and 73% of the theoretical yield, respectively). This is the first report of direct ethanol production from natural starchy substrates (without any pre-treatment or commercial enzyme addition) using industrial yeast strains co-secreting both a glucoamylase and α-amylase.

Expression and evaluation of enzymes required for the hydrolysis of galactomannan

The cost-effective production of bioethanol from lignocellulose requires the complete conversion of plant biomass, which contains up to 30 % mannan. To ensure utilisation of galactomannan during consolidated bioprocessing, heterologous production of mannan-degrading enzymes in fungal hosts was explored. The Aspergillus aculeatus endo-β-mannanase (Man1) and Talaromyces emersonii α-galactosidase (Agal) genes were expressed in Saccharomyces cerevisiae Y294, and the Aspergillus niger β-mannosidase (cMndA) and synthetic Cellvibrio mixtus β-mannosidase (Man5A) genes in A. niger. Maximum enzyme activity for Man1 (374 nkat ml(-1), pH 5.47), Agal (135 nkat ml(-1), pH 2.37), cMndA (12 nkat ml(-1), pH 3.40) and Man5A (8 nkat ml(-1), pH 3.40) was observed between 60 and 70 °C. Co-expression of the Man1 and Agal genes in S. cerevisiae Y294[Agal-Man1] reduced the extracellular activity relative to individual expression of the respective genes. However, the combined action of crude Man1, Agal and Man5A enzyme preparations significantly decreased the viscosity of galactomannan in locust bean gum, confirming hydrolysis thereof. Furthermore, when complemented with exogenous Man5A, S. cerevisiae Y294[Agal-Man1] produced 56 % of the theoretical ethanol yield, corresponding to a 66 % carbohydrate conversion, on 5 g l(-1) mannose and 10 g l(-1) locust bean gum.

Raw starch conversion by Saccharomyces cerevisiae expressing Aspergillus tubingensis amylases

BACKGROUND

Starch is one of the most abundant organic polysaccharides available for the production of bio-ethanol as an alternative transport fuel. Cost-effective utilisation of starch requires consolidated bioprocessing (CBP) where a single microorganism can produce the enzymes required for hydrolysis of starch, and also convert the glucose monomers to ethanol.

RESULTS

The Aspergillus tubingensis T8.4 α-amylase (amyA) and glucoamylase (glaA) genes were cloned and expressed in the laboratory strain Saccharomyces cerevisiae Y294 and the semi-industrial strain, S. cerevisiae Mnuα1. The recombinant AmyA and GlaA displayed protein sizes of 110-150 kDa and 90 kDa, respectively, suggesting significant glycosylation in S. cerevisiae. The Mnuα1[AmyA-GlaA] and Y294[AmyA-GlaA] strains were able to utilise 20 g l-1 raw corn starch as sole carbohydrate source, with ethanol titers of 9.03 and 6.67 g l-1 (0.038 and 0.028 g l-1 h-1), respectively, after 10 days. With a substrate load of 200 g l-1 raw corn starch, Mnuα1[AmyA-GlaA] yielded 70.07 g l-1 ethanol (0.58 g l-1 h-1) after 120 h of fermentation, whereas Y294[AmyA-GlaA] was less efficient at 43.33 g l-1 ethanol (0.36 g l-1 h-1).

CONCLUSIONS

In a semi-industrial amylolytic S. cerevisiae strain expressing the A. tubingensis α-amylase and glucoamylase genes, 200 g l-1 raw starch was completely hydrolysed (saccharified) in 120 hours with 74% converted to released sugars plus fermentation products and the remainder presumably to biomass. The single-step conversion of raw starch represents significant progress towards the realisation of CBP without the need for any heat pretreatment. Furthermore, the amylases were produced and secreted by the host strain, thus circumventing the need for exogenous amylases.

Comparative secretome analysis of Trichoderma asperellum S4F8 and Trichoderma reesei Rut C30 during solid-state fermentation on sugarcane bagasse

BACKGROUND

The lignocellulosic enzymes of Trichoderma species have received particular attention with regard to biomass conversion to biofuels, but the production cost of these enzymes remains a significant hurdle for their commercial application. In this study, we quantitatively compared the lignocellulolytic enzyme profile of a newly isolated Trichoderma asperellum S4F8 strain with that of Trichoderma reesei Rut C30, cultured on sugarcane bagasse (SCB) using solid-state fermentation (SSF).

RESULTS

Comparison of the lignocellulolytic enzyme profiles of S4F8 and Rut C30 showed that S4F8 had significantly higher hemicellulase and β-glucosidase enzyme activities. Liquid chromatography tandem mass spectrometry analysis of the two fungal secretomes enabled the detection of 815 proteins in total, with 418 and 397 proteins being specific for S4F8 and Rut C30, respectively, and 174 proteins being common to both strains. In-depth analysis of the associated biological functions and the representation of glycoside hydrolase family members within the two secretomes indicated that the S4F8 secretome contained a higher diversity of main and side chain hemicellulases and β-glucosidases, and an increased abundance of some of these proteins compared with the Rut C30 secretome.

CONCLUSIONS

In SCB SSF, T. asperellum S4F8 produced a more complex lignocellulolytic cocktail, with enhanced hemicellulose and cellobiose hydrolysis potential, compared with T. reesei Rut C30. This bodes well for the development of a more cost-effective and efficient lignocellulolytic enzyme cocktail from T. asperellum for lignocellulosic feedstock hydrolysis.

Phytase activity in Cryptococcus laurentii ABO 510

Ten Cryptococcus strains were screened for phytase activity, of which the Cryptococcus laurentii ABO 510 strain showed the highest level of activity. The cell wall-associated enzyme displayed temperature and pH optima of 62 degrees C and 5.0, respectively. The enzyme was thermostable at 70 degrees C, with a loss of 40% of its original activity after 3 h. The enzyme was active on a broad range of substrates, including ATP, D-glucose 6-phosphate, D-fructose 1,6-diphosphate and p-nitrophenyl phosphate (p-NPP), but its preferred substrate was phytic acid (K(m) of 21 microM). The enzyme activity was completely inhibited by 0.5 mM inorganic phosphate or 5 mM phytic acid, and moderately inhibited in the presence of Hg(2+), Zn(2+), Cd(2+) and Ca(2+). These characteristics suggest that the Cry. laurentii ABO 510 phytase may be considered for application as an animal feed additive to assist in the hydrolysis of phytate complexes to improve the bioavailability of phosphorus in plant feedstuff.

Cloning, characterisation, and heterologous expression of the Candida utilis malic enzyme gene

The Candida utilis malic enzyme gene, CME1, was isolated from a cDNA library and characterised on a molecular and biochemical level. Sequence analysis revealed an open reading frame of 1,926 bp, encoding a 641 amino acid polypeptide with a predicted molecular weight of approximately 70.2 kDa. The inferred amino acid sequence suggested a cytosolic localisation for the malic enzyme, as well as 37 and 68% homologies with the malic enzymes of Schizosaccharomyces pombe and Saccharomyces cerevisiae, respectively. Expression of the CME1 gene was subject to carbon catabolite repression and substrate induction, similar to the regulatory mechanisms observed for the C. utilis dicarboxylic acid permease. The CME1 gene was successfully expressed in S. cerevisiae under control of the S. cerevisiae PGK1 promoter and terminator. When coexpressed with the S. pombe malate permease gene (mae1), it resulted in a recombinant S. cerevisiae strain able to completely degrade 90% of the extracellular L-malate within 24 h.

Microbiology of a biological contactor for winery wastewater treatment

Winery wastewaters are characterised by large seasonal fluctuations in volume and composition and are often discarded with little or no treatment. A rotating biological contactor (RBC) was used to investigate microorganisms associated with the biological treatment of winery wastewater. Extensive biofilms developed on the RBC discs and contained a number of yeast and bacterial species that displayed a dynamic population shift during the evaluation period. This suggested that the naturally occurring microorganisms were able to form a stable biofilm and also reduce the chemical oxygen demand (COD) of winery wastewater (on average 43% with a retention time of 1h). One of the yeast isolates, MEA 5, was able to reduce the COD of synthetic wastewater by 95% and 46% within 24h under aerated and non-aerated conditions, respectively. The yeast isolates could therefore play an important role in the degradation of organic compounds under aerobic conditions, such as those associated with an RBC.

Malo-ethanolic fermentation in Saccharomyces and Schizosaccharomyces

Yeast species are divided into the K(+) or K(-) groups, based on their ability or inability to metabolise tricarboxylic acid (TCA) cycle intermediates as sole carbon or energy source. The K(-) group of yeasts includes strains of Saccharomyces, Schizosaccharomyces pombe and Zygosaccharomyces bailii, which is capable of utilising TCA cycle intermediates only in the presence of glucose or other assimilable carbon sources. Although grouped together, these yeasts have significant differences in their abilities to degrade malic acid. Typically, strains of Saccharomyces are regarded as inefficient metabolisers of extracellular malic acid, whereas strains of Sch. pombe and Z. bailii can effectively degrade high concentrations of malic acid. The ability of a yeast strain to degrade extracellular malic acid is dependent on both the efficient transport of the dicarboxylic acid and the efficacy of the intracellular malic enzyme. The malic enzyme converts malic acid into pyruvic acid, which is further metabolised to ethanol and carbon dioxide under fermentative conditions via the so-called malo-ethanolic (ME) pathway. This review focuses on the enzymes involved in the ME pathway in Sch. pombe and Saccharomyces species, with specific emphasis on the malate transporter and the intracellular malic enzyme.

Differential malic acid degradation by selected strains of Saccharomyces during alcoholic fermentation

To produce a high-quality wine, it is important to obtain a fine balance between the various chemical constituents, especially between the sugar and acid content. The latter is more difficult to achieve in wines that have high acidity due to excess malic acid, since wine yeast in general cannot effectively degrade malic acid during alcoholic fermentation. An indigenous Saccharomyces paradoxus strain RO88 was able to degrade 38% of the malic acid in Chardonnay must and produced a wine of good quality. In comparison, Schizosaccharomyces pombe strain F effectively removed 90% of the malic acid, but did not produce a good-quality wine. Although commercially promoted as a malic-acid-degrading wine yeast strain, only 18% of the malic acid was degraded by Saccharomyces cerevisiae Lalvin strain 71B. Preliminary studies on the transcriptional regulation of the malic enzyme gene from three Saccharomyces strains, i.e. S. paradoxus RO88, S. cerevisiae 71B and Saccharomyces bayanus EC1118, were undertaken to elucidate the differences in their ability to degrade malic acid. Expression of the malic enzyme gene from S. paradoxus RO88 and S. cerevisiae 71B increased towards the end of fermentation once glucose was depleted, whereas no increase in transcription was observed for S. bayanus EC1118 which was also unable to effectively degrade malic acid.

Malo-ethanolic fermentation in grape must by recombinant strains of Saccharomyces cerevisiae

Recombinant strains of Saccharomyces cerevisiae with the ability to reduce wine acidity could have a significant influence on the future production of quality wines, especially in cool climate regions. L-Malic acid and L-tartaric acid contribute largely to the acid content of grapes and wine. The wine yeast S. cerevisiae is unable to effectively degrade L-malic acid, whereas the fission yeast Schizosaccharomyces pombe efficiently degrades high concentrations of L-malic acid by means of a malo-ethanolic fermentation. However, strains of Sz. pombe are not suitable for vinification due to the production of undesirable off-flavours. Heterologous expression of the Sz. pombe malate permease (mae1) and malic enzyme (mae2) genes on plasmids in S. cerevisiae resulted in a recombinant strain of S. cerevisiae that efficiently degraded up to 8 g/l L-malic acid in synthetic grape must and 6.75 g/l L-malic acid in Chardonnay grape must. Furthermore, a strain of S. cerevisiae containing the mae1 and mae2 genes integrated in the genome efficiently degraded 5 g/l of L-malic acid in synthetic and Chenin Blanc grape musts. Furthermore, the malo-alcoholic strains produced higher levels of ethanol during fermentation, which is important for the production of distilled beverages.

Factors involved in the regulation of the Schizosaccharomyces pombe malic enzyme gene

Transcription of the Schizosaccharomyces pombe malic enzyme gene, mae2, is induced when cells are grown on high glucose concentrations or under nonaerated conditions. Two cis-acting elements in the mae2 promoter, upstream activator sequences UAS1 and UAS2, are required for basal expression, whilst three negative-acting, upstream repressor sequences are involved in general derepression of mae2. Both the Pka1 and Sty1 signal transduction pathways are involved in the induced expression of mae2 under fermentative conditions. Expression of mae2 seems to be regulated in response to the carbon source, lack of oxygen and osmotic stress conditions, probably to assist in maintaining the intracellular redox balance.

Differential uptake of fumarate by Candida utilis and Schizosaccharomyces pombe

The dicarboxylic acid fumarate is an important intermediate in cellular processes and also serves as a precursor for the commercial production of fine chemicals such as L-malate. Yeast species differ remarkably in their ability to degrade extracellular dicarboxylic acids and to utilise them as their only source of carbon. In this study we have shown that the yeast Candida utilis effectively degraded extracellular fumarate and L-malate, but glucose or other assimilable carbon sources repressed the transport and degradation of these dicarboxylic acids. The transport of both dicarboxylic acids was shown to be strongly inducible by either fumarate or L-malate while kinetic studies suggest that the two dicarboxylic acids are transported by the same transporter protein. In contrast, Schizosaccharomyces pombe effectively degraded extracellular L-malate, but not fumarate, in the presence of glucose or other assimilable carbon sources. The Sch. pombe malate transporter was unable to transport fumarate, although fumarate inhibited the uptake of L-malate.